(1) Field of the Invention
The present invention relates to a model reference following commutation circuit and an adjusting method thereof, and in particular, to a model reference following commutation circuit for driving a brushless direct current motor and an adjusting method thereof, which optimally controls a commutation delay time and a soft switching operation to mask spurious zero-cross detections and to optimize motor performance according to individual applications.
(2) Description of Related Art
In conventional circuits for driving brushless and sensorless direct current (DC) motors, it is generally desired that an actual back-emf zero-cross point be located out of phase from an commutation switching point. For a typical three-phase DC motor, the commutation switching point is ideally located 30 eletrical degrees from the zero-cross point. Advanced methods are needed to find the optimal switching point in order to drive a motor effectively.
In addition, multi-phase DC motors have induced load characteristics which are represented by a motor electrical constant, L/R. in general, because the time constant of a motor is longer than switching time of an electric device used to control commutation in the motor, such as a field effect transistor (FET), this time difference may cause switching noise, called spikes, and may even cause current re-circulation back to a power line. Moreover, spikes may adversely cause a comparator in a circuit to detect a false zero-cross of the back-emf. Therefore, masking is required to get rid of this disturbance.
Furthermore, soft switching is needed to adjusting a turn on/off time of a transistor controlling commutation and to eliminate the disturbance caused by spikes. As shown in FIGS. 1A and 1B, one can adjust the turn on/off time so that commutation starts from a certain point before the center point and continues to a certain point after the center point, and through this commutation a soft switching can be performed. In other words, the center point is ideally halfway through a commutation interval and aligned with a center reference axial, so that it is located symmetrically with the starting point and extended point at the left and right sides of the center reference axial.
Related art exists for finding an optimal switching point for commutation and for masking fake zero-cross points incorrectly detected due to noise caused by phase commutation. Techniques for detecting an optimal switching point and masking false detections using three counters are disclosed in U.S. Pat. No. 5,221,881, issued Jun. 22, 1993, and entitled "Method and Apparatus for Operating Polyphase DC Motors" and U.S. Pat. No. 5,317,243, issued May 31, 1994, and entitled "Method and Apparatus for Detecting Velocity Profiles of a Spinning Motor of a Polyphase DC Motor."
Among the three counters used in the related art, one is an up-counter that counts the zero-cross period. The other two are down-counters of which the first down-counter counts down to zero to generate a commutation delay signal; the second down-counter starts counting down after the first down-counter completed its counting to generate a masking time. When a zero-cross is detected, the counting result of the up-counter is loaded into both of the first and the second down-counters, then the up-counter is rested until a new commutation period.
Although the digital counters used in the above techniques are an advanced approach, there are four problems worth noting. The first problem is that the up-counter must load the counting results to the down-counter, and loading errors may occur.
The second problem is that the up-counter requires operations such as stopping, loading, resetting, and restarting of the counting, all of which are time-consuming. Therefore, these operations are performed in an off-line state and not in a real time signal process to prevent losing the track of the disk and to provide accurate control.
The third problem is that four different signals are needed to consecutively execute the procedures. However, these cannot be performed at the same time. If the pulse width of a control signal is 0.01 .mu.s, from stop counting to restart counting, approximately 0.04 .mu.s is required to issue the signals. This 0.04 .mu.s is wasted, and when performed off-line, may result in lost tracking. Furthermore, miscommunication for 0.04 .mu.s is caused while the period counter counts zero-crossings, which affects the rotation speed.
And the fourth problem is that it is difficult to find the rate of acceleration for the motor. For this purpose, SGS-Thomson uses an additional N+1 bits counter to control the clock frequency for the mask and delay counters based on the detected rate of acceleration. However, this is almost useless in high-density disk drive (HDD) applications, because in steady states, the motor speed is locked up to be quite stable, and the deviation of rotor velocity is typically less than 0.05%, which is practically negligible and difficult to measure.
In order to solve these problems, a conventional circuit, using a delay including at least one resistor-capacitor, is disclosed in U.S. Pat. No. 5,233,275, issued Aug. 3, 1993, and entitled "A Simplified Sensorless DC Motor Commutation Control Circuit Using Analog Timing Techniques."
However, as is known, once a resistor-capacitor RC constant time is set, there is no way that one can change this time constant during motor operations. For both low and high frequency motor rotation speeds, this fixed time constant may be neither sufficient nor appropriate for the dynamic motor commutation. In other words, a user must always take the trouble to find an optimal value for their type of motors. That includes the difficulty in deciding upon a set-of values for the resistor and capacitor according to the un-unified motor parameters.
Accordingly, to solve the above-mentioned problems, a circuit for adjusting the commutation delay time comprising a feedback circuit between the motor and the driver, is introduced in U.S. Pat. No. 5,285,135, issued Feb. 8, 1994, and entitled "Automatic Adjustment of Commutation Delay for Brushless DC Motor for Improved Efficiency." This circuit adjusts the commutation delay by performing a transfer function to modify a signal for adjusting the delay time. However, detecting the center position using that circuit without a position sensor is not an easy task. And, if a position sensor or other hardware is used, it could make the drive circuit more complicated in terms of the electronic components of the drive IC either internally or externally.